S2 Reactions with an Ambident Nucleophile: A Benchmark Ab Initio Study of the CN + CHY [Y = F, Cl, Br, and I] Systems.

We characterize the Walden-inversion, front-side attack, and double-inversion S2 pathways leading to Y + CHCN/CHNC and the product channels of proton abstraction (HCN/HNC + CHY), hydride-ion substitution (H + YHCCN/YHCNC), halogen abstraction (YCN/YNC + CH and YCN/YNC + CH), and YHCN/YHNC complex formation (YHCN/YHNC + CH) of the CN + CHY [Y = F, Cl, Br, and I] reactions. Benchmark structures and frequencies are computed at the CCSD(T)-F12b/aug-cc-pVTZ level of theory, and a composite approach is employed to obtain relative energies with sub-chemical accuracy considering (a) basis-set effects up to aug-cc-pVQZ, (b) post-CCSD(T) correlation up to CCSDT(Q), (c) core correlation, (d) relativistic effects, and (e) zero-point energy corrections. C-C bond formation is both thermodynamically and kinetically more preferred than N-C bond formation, though the kinetic preference is less significant.